The present disclosure is related to reducing defects which form during growth of semiconductor materials over a substrate, and more specifically to reducing certain dislocations which arise due to the lattice mismatch between a substrate and a nitride material grown thereover.
It is quite well known that when epitaxially growing a material such as a semiconductor over a substrate, a mismatch in the lattice constants of the substrate and the growth material can result in crystalline defects in the material as grown. This is illustrated in FIG. 7 for a structure 100. Substrate 102 is typically oriented such that its crystal boundaries 104 are oriented roughly perpendicular to the plane of the growth surface (i.e., vertically). As a growth material 106, such as gallium nitride (GaN), forms over substrate 102, lattice defects 108 form therein which are roughly parallel to the crystal boundaries 104 of substrate 102. There are various techniques known in the art for suppressing these defects.
One material system of particular interest today is the nitride system (e.g. compounds formed between any group-III element and nitrogen). Nitride-based materials are able to produce light-emitting devices such as diode lasers and the like which emit light at shorter wavelengths corresponding to green, blue, and even ultra violet (UV) light as compared to other known material systems. Other applications for the nitrides are transistors and other electronic devices. However, nitride materials are typically grown by metalorganic chemical vapor (MOCVD) deposition techniques onto lattice-mismatched substrates like for example sapphire, silicon carbide, and silicon for which there are relatively fewer options to address and prevent lattice mismatch defects.
One known technique for suppressing lattice dislocations in MOCVD processes is known as lateral overgrowth, which is illustrated in FIG. 8. Again, a c-plane oriented substrate 102 such as Al2O3 (sapphire), is the typical starting point. In order to suppress the vertical dislocation defects, a mask layer 112 is formed over the surface of substrate 102, and one or more openings 114 are then formed in the mask. GaN layer 116 is then epitaxially grown over substrate 102, beginning in openings 114 in order to initiate the epitaxy. The GaN grows both vertically and laterally. Due to the crystallographic orientation of substrate 102, any vertical defects forming in layer 116 are limited to the opening areas, and are either suppressed or bent horizontally over the mask 112. Thus the regions over the mask 112 are substantially free of vertical lattice dislocations.
While c-plane oriented substrates have been the most widely used substrates to date, other orientations such as semi-polar and m-plane orientations are becoming increasingly important. For example, bulk semi-polar GaN substrates are highly desired for (InGaAl)N-based light emitters such as light-emitting diodes (LEDs) and laser diodes (LDs), in order to reduce internal electric fields which impair the efficiency of the light emission process on conventional c-axis oriented nitride devices. However, such bulk substrates are not yet widely available and are limited to small sizes. As an alternative to bulk semi-polar GaN substrates, semi-polar GaN templates have been grown on large area sapphire substrates by conventional means such as Hydride Vapor Phase Epitaxy (HVPE). However, the defect density in such template layers is of the order of 1010 cm−2, unless defect reduction techniques are applied.
While lateral overgrowth is an effective technique for c-plane oriented substrates, it is not optimized for materials in which the c-axis is tilted with respect to the surface normal, such as any semi-polar oriented GaN, in which a significant portion of defects extend across the GaN layer at an angle corresponding to the tilt of the basal plane GaN(0001). With reference next to FIG. 9, one difficulty observed is that since the lattice defects 128 in semi-polar template layer 122 (or equivalently, a semi-polar substrate, not shown) run diagonally, e.g., at a given angle between 0 and 90 degrees relative to the plane of growth surface 132 of layer 122, the effectiveness of the mask at limiting communication of the defects into the growth layer 116 is reduced. FIGS. 10a-10f are TEM images of GaN(1122) layers grown by a lateral overgrowth technique on a semi-polar buffer layer, as known in the art. Cross-section images shown in FIGS. 10e and 10f highlight the persistence of diagonally running defects despite the presence of lateral overgrowth mask 126. To compound this problem, certain substrate orientations present defects in multiple different planes (e.g., perpendicular to the growth surface as well as angled relative to that plane). To date, there have been inadequate solutions for suppressing lattice defects in epitaxial growth layers formed over non-c-plane oriented layers.